I. Field of the Invention
The present invention pertains to a new ball circulating system for linear guide assembly that will improve the smoothness of ball circulation of linear guide assembly.
II. Description of the Prior Art
Linear guide assembly has been extensively used in sophisticated machines and instruments. Linear guide assembly comprises of a finite straight rail, a set of steel balls and a sliding block which includes a block body, two end plates, two end seals, some steel ball retainers, and other accessories. While the sliding block and the rail move relative to each other, the circulating steel balls transfer the force between the sliding block and the rail. Meanwhile the slide friction only exists on the end seals but not on the other parts, so that the frictional force between the sliding block and the rail is virtual small.
Conventional linear guide assembly has several rows of arc-like grooves on block body. Those grooves where steel balls circulate between the rail and sliding block are parallel to rail. The passage that the steel balls circulate along the above mentioned grooves and bear the load is so called loading passage. There are also several tunnels in block body. These tunnels are parallel to each corresponding loading passage, and allow the steel balls circulating in between but without enduring any load. Then they are called unloading passages. In order to let steel balls circulate continuously, both ends of loading passage are connecting to a semicircular-like tunnel which is called connecting passage. Through the connecting passage, steel balls can circulate from loading passage to unloading passage and back to loading passage by through another connecting passage. Therefore, steel balls circulate continuously.
Conventional linear guide assembly's circulating system is shown in FIG. 6. There is no sliding motion between steel balls (8), sliding block (10) (which includes block body (1) and end plates (3)) and rail (2). The steel ball (8) on the loading passage (4) is moving at the center velocity of V/2 to the left relative to the rail (2) when the sliding block (10) moves to the left at the velocity of V relative to the rail (2). Meanwhile, the steel balls have an angular velocity of V/2r counterclockwise. Since the steel ball (8) moves to the right with a velocity of V/2 relative to the sliding block (10), the steel balls (8) circulate along the loading passage (4) into an intersection point (a) which is between loading passage (4) and the connecting passage (5). The steel balls (8) strike the connecting tunnel's lip edge (7) which is called directional guiding lip (7). If the steel balls circulate in the connecting passage and contact with the outer surface of the tunnel without relative sliding motion, the steel balls' angular velocity is V/2r in clockwise direction, but it is in the counterclockwise direction during the loading passage (4). Near the directional guiding lip, the angular velocity and kinetic energy of the steel balls change rapidly in order to reduce frictional resistance, so the directional guiding lip (7) will bear a large force. When the steel balls (8) strike at the directional guiding lip (7), there is a large clockwise torque, a leftward and upward force. As a result, these forces will cause the directional guiding lip to wear as well as induce noises and increase the frictional resistance of linear guide assembly. Due to the kinetic energy is proportional to the square of the velocity, the noise and sliding resistance is increased by exponent when the moving velocity of the sliding block increase. In order to improve the production efficiency, the sliding block's speed will be increased continuously. Therefore the striking problem of the directional guiding lip will become much more severe.
Due to the centrifugal effect, the steel balls usually contact with the outer circularity (5a) of the tunnel when they circulate in the connecting passage (5). The steel balls are sliding and rolling relative to the outer circularity of the connecting tunnel. Because of the centrifugal force of the steel ball is proportional to the square of the velocity, if the moving velocity of the sliding block relative to the rail increases; the normal force exerted on the steel ball and the surface of outer circularity (5a) will increase exponentially. Besides, the coefficient of sliding friction of the steel ball and the connecting tunnel is usually much larger than steel balls themselves. Most of the resistance between the steel ball and the connecting tunnel is rolling friction, so the sliding friction is relatively small portion.
Two conventional connecting passages are shown in FIG. 7 and FIG. 8. FIG. 7 is the oldest design in which the center of steel ball (8) circulates along a semicircular tunnel; both loading passages (4) and unloading passage (6) are straight and connecting passage (5) is a semi-circular. The modified type of FIG. 7 as shown in FIG. 8 is to modify the semi-circular passage (5) of FIG. 7 into a combination of two quarter-circular (52) and a tangential straight passage (51).
Two types of circulating passage mentioned above have been studied quite a lot in U.S. Pat. No. 4,505,522. However, the patent focuses on the effect of the amount of steel balls circulating in the connecting tunnel on the frictional resistance of linear guide. There is no discussion of the problem concerning about steel balls striking the directional guiding lip. The designs of these two traditional connecting tunnels are tangential to loading passage in order to reduce the striking resistance by the steel ball as shown in FIG. 6. However, since the radius of curvature of directional guiding lip in the connecting passage is not infinite, the steel balls at directional guiding lip are under a large force. As a result, the normal force of steel balls exerted on directional guiding lip will increase and so will the relative friction. The friction between steel balls and directional guiding lip is sliding friction instead of the smaller rolling friction. Therefore, if sliding block moves rapidly relative to rail, steel balls will understate a very large striking force. Besides, if the groove in rail of conventional linear guide of circulating ball is arc-shaped and directional guiding lip is tangential to loading groove, the tip of directional guiding lip will become very sharp and easy to be broken down. Directional guiding lip can not contact with rail directly and there is a seam in between.
FIG. 9 is an enlargement figure of the surrounding of directional guiding lip (7). From that figure, it can be seen that there is a discontinuous bumping when steel balls (8) circulate from loading passage (4) into connecting tunnel (5). The discontinuity will consequently increase the noise and frictional resistance of linear guide assembly of circulating ball.
Mr. Geka in U.S. Pat. No. 4,652,147 proposed another two types of connecting passages. As shown in FIG. 10, the first type modified the semicircular shape of connecting passage (5) to a combination of several tangential arcs. Three arcs constitute the upper right of FIG. 10. The radius of the first arc is R1. The radius of the second arc is R2. The radius of the third arc is R3. The second arc is tangential to the first and third arc respectively. R2 is obviously a lot smaller than R1 and R3. As shown in FIG. 10, the R2 is designed to be moved to the upper right and the lower left portion of the connecting passage to avoid the interference for the steel ball's circulation. Therefore, the radius of curvature near the directional guiding lip can have more space. As shown in FIG. 11, the second type modified conventional semicircular shape of connecting tunnel to a half ellipse or a combination of two quarter ellipses (54) and a tangential line (53).
"A" denotes the radius of major axis of ellipse. "B" denotes the radius of minor axis of ellipse. Ellipse can be denoted as the following formula: EQU X.sup.2 /A.sup.2 +Y.sup.2 /B.sup.2 =1 . . . (1)
The radius of curvature of the major axis's top point is B.sup.2 / A which is the minimum value. The radius of curvature of the minor axis's top point is A.sup.2 /B which is the maximum value. The top point of minor axis of ellipse in FIG. 11 is located at directional guiding lip of connecting passage in order to reduce the normal force exerted on directional guiding lip.
The characteristics of the above modification is that the top points of major axis are the smallest radius of curvature and that of minor axis is the largest radius of curvature as shown in FIG. 11. Meanwhile the top point of major axis (the smallest radius of curvature) is on the upper right and lower right of FIG. 11 and directional guiding lip on the top point of minor axis (the largest radius of curvature). Although both the first and second modifications have enlarged the radius of curvature of connecting tunnel near directional guiding lip and thus reduced noise and frictional resistance of linear guide assembly, the two basic problems still exist in conventional linear guide: the tip of directional guiding lip still undertakes centrifugal force and the discontinuous bumping phenomena of steel balls are still present.
In order to reduce the size of the sliding block, the connecting tunnel of linear guide of circulating ball is sometimes not located at the plane but on a curvature plane. As shown in FIG. 12 (U.S. Pat. No. 4,610,488), the center of connecting passage (5) in sideview is not a straight line but a curve Q--Q'. The length of Q--Q' curve is equal to 2 B of equation (1).
The steel ball (8) which circulates into connecting passage (5) from loading passage (4) can be clearly seen in FIG. 9. At the intersection point (a) of the loading passage and the connecting passage, the steel balls have not been changed the circulating direction by the directional guiding lip (7). The steel ball keeps on moving along with the loading groove (4) until strike to the tip of directional guiding lip. As a result, steel ball bumps discontinuously to induce noise and increase the frictional resistance. FIG. 13 and FIG. 14 are two types of side view of FIG. 9. In these two circulating ways, the seam between the steel ball (8) and the steel ball groove (2a) is so small that the directional guiding lip (7) becomes very sharp. In addition, there is quite a distance from the tip of the directional guiding lip to the contact surface of block body (1) and end plate (3). Therefore, there is a section that steel ball circulates without any guiding. As shown in FIG. 13, the circulating direction of steel balls is perpendicular to the contact face of steel balls and groove. As shown in FIG. 14, there is one kind of design (such as U.S. Pat. No. 5,108,197) in which the angle between the steel balls circulating direction and the contact face's normal vector direction is under 60 degrees in order to reduce the size. The clearance between the steel ball and the groove is quite small so that either the tip of directional guiding lip becomes very sharp or the steel ball bumps discontinuously as shown in FIG. 9. The cross sections of steel ball groove on rail 2a and 2a' in FIG. 13 and FIG. 14 respectively indicate that their arc lengths are very long. However, the effective range to sustain the steel ball's loading is the contact point between the steel ball and the groove. The range of the contact point is .theta. as shown in FIG. 13 and 14 and the effective area is only within 45 degree. The tip of directional guiding lip is still very sharp and the steel ball still bumps discontinuously. As shown in FIG. 13, it can not machine an escaped groove on the efficient range of groove to undertake the steel ball's loading because the steel ball's circulating direction is perpendicular to the contact surface between the steel ball and groove or the angle between the steel ball's circulating direction and the contact surface's normal vector direction is not large enough.